How to make glass more reflective?
The correct way to add reflective coating is to mix in a Glossy shader through Fresnel:



This is what it does - it coats the surface with more reflection, becoming fully reflective at grazing angle (it is influenced by the Fresnel IOR):



The result is less-transparent more-reflective, denser looking glass. Which can be done with glass IOR itself as glass already has reflection:)

But we can apply this generally to any shader, to add a reflective coating (clear-coat) on top. Such coating is usually so thin it is fully transparent and only it's reflective properties matter (the reflection is of dielectric nature - so white).

This has also the benefit of ability to change the base reflectance of the coating if really needed. Here, let's have base reflectance just 20% of the normal Fresnel node (also note that base reflectance of dielectrics don't change much):